Once the brown marmorated stink bug eggs have been parasitized, the samurai wasp chews its way out, leaving a jagged hole.
A natural enemy has arrived to help control the brown marmorated stink bug, an invasive pest that devastates gardens and crops, particularly orchard fruits and nuts.
The samurai wasp (Trissolcus japonicus) is a tiny insect that hunts for the egg masses of brown marmorated stink bugs (BMSB) and lays an egg inside each egg in the mass. Since there are 28 eggs in a cluster, that’s the potential for 28 more wasps. The parasitic wasp develops inside the egg, effectively killing the stink bug, and then chews its way out.
The brown marmorated stink bugs, which showed up in Oregon in 2004, feed on more than 100 plants, particularly vegetables, pears, apples and hazelnuts, but also ornamentals. The economic damage to commercial crops in Oregon hasn’t yet been determined, but is substantial in other parts of the country, said David Lowenstein, an Oregon State University Extension Service entomologist. The damage done by brown marmorated stink bugs to crops and garden plants makes discovery of the samurai wasp all the more important, he added.
So far, tests to determine the success of the samurai wasp against the invasive stink bugs have been positive, said Lowenstein, who was the lead author on a guide to identifying the samurai wasp.
“The wasp is a better way to control BMSB because it’s a biological control agent that reduces the need for chemicals — which are only somewhat effective — and it’s a specialist,” he said. “It doesn’t lay eggs in other insect eggs except those of other stink bugs.”
Homeowners must deal with the added nuisance of the shield-shaped bugs invading houses, especially in fall when they’re looking for a place to overwinter.
“Brown marmorated stink bugs are unmistakable because of the stinky, irritating odor when they’re crushed,” said Vaughn Walton, an entomologist with OSU Extension. “They move from wild host plants to our gardens and then in large amounts into our homes. That’s when people get really upset. Bugs inside freak people out.”
Control is difficult, however. Research has shown that pesticides are only a short-term solution and will kill other beneficial insects, including the samurai wasp.
Brown marmorated stink bugs can be distinguished from other stink bugs by the bands on their antennae. In spring, adults start eating and laying eggs on the undersides of leaves. Within a week, the eggs hatch into immature bugs and eventually adults and the process starts again. The bug could reproduce up to three times a year depending on conditions, said Vaughn Walton, an entomologist with OSU Extension who helped produce a guide on the invasive pests.
The wasp is native to East Asia, the same range as the brown marmorated stink bugs, so is a natural enemy and probably hitched a ride on a ship. Scientists at OSU, including Lowenstein, had already begun rearing and studying the beneficial insect in the lab when it was found last year in 11 areas in the Willamette Valley, including several in Portland. Since none had been released from the lab, the discovery is significant.
This summer Lowenstein and other researchers will revisit those areas and some nearby to see if the wasp survived the winter.
“Some natural enemy controls do great in the lab, but don’t perform as well in the field,” he said. “But the fact that it’s already widespread across Portland is a positive sign that it can withstand conditions in Oregon.”
Wooded areas, particularly those bordering orchards, are most likely to serve as habitat for the wasp, which can move quickly over long distances. But since the stink bugs have migrated into urban locations, experts expect the wasps to survive there, as well.
The public can serve a key role in samurai wasp research by collecting possible brown marmorated stink bug egg masses and reporting them to Lowenstein on a form at the Wiman Lab website, where more information, tips and photos are available to help with identification. In short, look for black eggs, which means they’ve been parasitized. Once the wasps emerge, there will be irregular holes. Taking photos is strongly encouraged.